Fig. 6. LR defects in Pitx2^ASE/ASE mice. Visceral organs and heart sections of wild-type (+/+) and Pitx2^ASE/ASE neonates are shown in A-H,M-V. (A,B) The left and right lungs of wild-type mice have one and four lobes (indicated in yellow), respectively, whereas the lungs of all Pitx2^ASE/ASE mice had four lobes on each side. (C,D) The relative positions of the aorta (ao, blue asterisk) and pulmonary artery (pa, yellow asterisk) are reversed in Pitx2^ASE/ASE mice. The position of the heart apex is toward the left in wild-type mice but was reversed (toward the right) in 28% of Pitx2^ASE/ASE mice. The inferior vena cava is present only on the right side of the thorax in wild-type mice (C, blue arrowhead) but was bilateral in 28% of Pitx2^ASE/ASE mice (D, two arrowheads). (E,F) External morphology of the atrium showing right isomerism in Pitx2^ASE/ASE mice. In wild-type mice, the left atrium has a narrow junction to the venous component (green line in E), whereas the right atrium has a wide junction. In Pitx2^ASE/ASE mice, both the left (green line in F) and right atria exhibited a morphology similar to that of the right atrium of the wild type. (G,H) Frontal sections of the heart. Atrial septal defect was apparent in the mutant heart. (I-L) AV cushions of the heart at E12.5 are shown by scanning electron microscopy (SEM) (I,J) and by sagittal sectioning of HE-stained hearts (K,L). SAVC and IAVC are fused with each other in the wild-type heart, but they remain separated in the Pitx2^ASE/ASE embryos. The yellow arrowheads in K denote the boundary between SAVC and IAVC. The dotted area in L indicates a portion of SAVC that is missing in the Pitx2^ASE/ASE embryos. (M,N) The azygos vein and thoracic aorta are located on the left side of the thorax in wild-type mice, whereas they were present on the right side in some Pitx2^ASE/ASE mice. (O,P) The portal vein passes dorsally to the duodenum and is not visible from the ventral side in wild-type mice, but it was on the ventral side of the duodenum in some Pitx2^ASE/ASE mice (black arrow in P). (Q-T) Abnormal rotation of the gut, including aberrant looping of the duodenum (S) or the lack of a cross of the duodenum and colon (T), was apparent in Pitx2^ASE/ASE mice at a high frequency. Blue arrows indicate positioning of the gut. The pancreas, which is located on the left side of the duodenum and is located behind the ileum in a ventral view in wild-type mice (Q), was positioned on the ventral or right side of the duodenum in Pitx2^ASE/ASE mice (R). (U,V) In the abdomen of wild-type mice, the aorta and vena cava are located on the left and right sides of the midline, respectively; their relative positions were reversed in 25% of Pitx2^ASE/ASE mice. Scale bars: 200 µm. ao, aorta; az, azygos vein; co, colon; du, duodenum; IAVC, inferior atrioventricular cushion; la, left atrium; lv, left ventricle; pc, pancreas; ra, right atrium; rv, right ventricle; SAVC, superior atrioventricular cushion; ta, thoracic aorta; vc, vena cava.